Polylactic acid shrink films and methods of manufacturing same

ABSTRACT

A heat-shrinkable polylactic acid (PLA) film and a method of its manufacture are provided. In an exemplary embodiment, the PLA films exhibit heat-induced growth in the cross direction with concomitant shrinkage in the machine direction. The films may comprise any grade of PLA polymer, optionally including additives, such as antiblock, slip, viscosity enhancers and combinations thereof. A method of manufacture is disclosed which includes a post-extrusion temperature conditioning step.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 60/672,515 filed Apr. 19, 2005, the disclosure of which has beenincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to heat-shrinkable film. Moreparticularly, the present invention relates to heat-shrinkablepolylactic acid films that exhibit temperature dependent shrinkage inone direction with concomitant expansion in another.

Heat-shrinkable films have widely been used for various industrialapplications such as, for example, with shrink-wrap films, shrinkablelabels and cap seals, by making use of their property of heat-dependentshrinkage. The films have been applied to a variety of surfaces,including plastic and glass surfaces. Shrink films have beenmanufactured from vinyl chloride resins, polystyrene resins or polyesterresins; however, in many instances, film shrinkage is concomitant withundesirable curling of the outer edges of the film. The undesirablecurling of the outer edges has limited the application of shrink films.Therefore, there is a need for shrink films that have little to nocurling.

As well, interest in compostable polymers, i.e. biopolymers, has greatlyincreased, and many companies have made efforts to market, for example,packaging materials, hygiene products, sacks, and films with compostablepolymers. Polylactic acid (PLA), i.e., polylactide, or condensationpolymers which are based on lactic acid, are for many reasons a veryattractive group of biopolymers. Their principal degradation product,lactic acid, is a product common in nature, it is not toxic and is usedwidely in the food and pharmaceutical industries.

While some manufacturers have resorted to manufacturing PLA film withcasting methodology (e.g., cast and tenner), the films therefrom havelimited applications and are 5 to 10 times more costly than blown filmprocessing. Accordingly, there remains a need for PLA blown films withdesirable shrink properties.

SUMMARY OF THE INVENTION

The foregoing needs are met, to an extent, by the present invention,wherein in one aspect shrink film is provided comprised of a PLA polymerblend which, upon exposure to heat, exhibits shrinkage in the lengthdirection and growth in the cross direction. The shrink film may exhibitshrinkage in the length direction in the range of about 10% to about 90%and exhibit a shrinkage in the cross direction in the range of about 0%to about −30% (a negative value indicates growth) when subjected to 95°C. heat for about 10 seconds. In some embodiments, the shrink film mayexhibit shrinkage in the length direction of about 50% and exhibitshrinkage in the cross direction of about −5% when subjected to 95° C.heat for about 10 seconds. In other embodiments, the shrink film mayexhibit shrinkage in the length direction of about 30% and exhibit ashrinkage in the cross direction of about −5% when subjected to 95° C.heat for about 10 seconds. In still yet other embodiments, the shrinkfilm may exhibit shrinkage in the length direction of up to about 50% orgreater and exhibit a shrinkage in the cross direction of up to about−10% when subjected to heat over a temperature range of about 60° C. toabout 95° C. after about 10 seconds.

The inventive PLA shrink films may further comprise one or moreadditives, such as an antiblock additive, a slip additive, a viscosityenhancer or a combination thereof. The antiblock additive may be naturalsilica, synthetic silica, talc, talc filled magnesium, calciumcarbonate, and N,N′-Ethylene Bis(Stearamide) (EBS). The preferredantiblock additive in some embodiments is talc filled magnesium. Theslip additive may be oleamide, erucamide, stearamide, behenamide, oleylpalmitamide, stearyl erucamide, ethylene bis-oleamide, EBS, or acombination thereof, and preferably EBS in some embodiments. Viscosityenhancers may include, for example, stabilizers or coupling agents. Apreferred coupling agent is CESA®-extend.

The PLA polymer blends of the inventive shrink films disclosed hereinmay include two or more “grades” of PLA polymer. For example, PLApolymer may comprise about 1 to about 2 percent by weight D-lactide;about 3 to about 5 percent by weight D-lactide; or about 11 to about 13percent by weight D-lactide. In some embodiments, the shrink films maycomprise about 50 weight percent to about 90 weight percent of PLApolymer having about 11 to about 13 percent by weight D-lactide; andabout 10 weight percent to about 50 weight percent of a PLA polymerhaving about 1 to about 2 percent by weight D-lactide. In otherembodiments, the shrink films may comprise about 60 weight percent toabout 80 weight percent of PLA polymer having about 11 to about 13percent by weight D-lactide; and about 20 weight percent to about 40weight percent of a PLA polymer having about 1 to about 2 percent byweight D-lactide. In yet other embodiments, the shrink films maycomprise about 65 weight percent to about 75 weight percent of PLApolymer having about 11 to about 13 percent by weight D-lactide; andabout 25 weight percent to about 35 weight percent of a PLA polymerhaving about 1 to about 2 percent by weight D-lactide.

Further, the present invention provides films comprising about 50 weightpercent to about 90 weight percent of PLA polymer having about 11 toabout 13 percent by weight D-lactide; about 10 weight percent to about50 weight percent of a PLA polymer having about 1 to about 2 percent byweight D-lactide; less than about 3 percent by weight of an antiblockadditive; less than about 1 percent of a slip additive; and less thanabout 0.5 percent of a viscosity enhancer. The inventive films may alsocomprise about 50 weight percent to about 90 weight percent of PLApolymer having about 11 to about 13 percent by weight D-lactide; about10 weight percent to about 50 weight percent of a PLA polymer havingabout 1 to about 2 percent by weight D-lactide; less than about 2percent by weight of an antiblock additive; less than about 0.5 percentof a slip additive; and less than about 0.25 percent of a viscosityenhancer. Preferably, in some embodiments, the inventive shrink filmscomprise about 50 weight percent to about 90 weight percent of PLApolymer having about 11 to about 13 percent by weight D-lactide; about10 weight percent to about 50 weight percent of a PLA polymer havingabout 1 to about 2 percent by weight D-lactide; less than about 1percent by weight of an antiblock additive; less than about 0.25 percentof a slip additive; and less than about 0.1 percent of a viscosityenhancer.

In accordance with another embodiment of the present invention, apackaged good is provided comprising a consumable item and a heattreated film wrapped around at least a portion of the consumable item,which heat treated film is obtained by exposing to heat a shrink filmcomprised of a PLA polymer blend which exhibits shrinkage in the lengthdirection and growth in the cross direction upon exposure to heat. Thepackaged good may be a consumable item, including batteries, cans,bottles, disposable lighters, pens and decorative items. The film mayform a perforated or non-perforated neck band around the consumable itemand may be clear, matte, translucent, or opaque. The packaged good withthe heat treated film may include a packaging label and the label may beprinted onto the film.

In yet another embodiment of the present invention, a method of making ashrink film comprising (a) providing pellets of PLA, (b) melting thepellets to form a molten mass at a first desired viscosity value orrange of values, (c) increasing the viscosity of the molten mass to asecond desired viscosity value or range of values, (d) forming a heatedbubble from the resulting molten mass, and (e) collapsing the bubble toform a film. The method may optionally include drying the pellets, forexample, in a dehumidifying hopper, prior to melting the pellets to forma molten mass.

The melting step may be carried out at a temperature may range fromabout 325° F. to about 485° F., preferably from about 375° F. to about425° F., and more preferably at about 400° F. In some embodiments, thefirst viscosity value ranges from about 1,000 P to about 5,000 P atabout 480° F. and an apparent shear rate of about 55 seconds⁻¹,preferably from about 2,000 P to about 4,000 P at about 480° F. and anapparent shear rate of about 55 seconds⁻¹, and more preferably about3,000 P at about 480° F. and an apparent shear rate of about 55seconds⁻¹. The second viscosity value may range from about 14,000 P toabout 16,000 P at about 375° F. and an apparent shear rate of about 55seconds⁻¹, preferably from about 15,500 P to about 16,500 P at about375° F. and an apparent shear rate of about 55 seconds⁻¹, and morepreferably about 15,000 P at about 375° F. and an apparent shear rate ofabout 55 seconds⁻¹. The viscosity increasing step may be carried out ina polymer cooling unit, internal cooling die mandrel, and/or with theaddition of chemical viscosity enhancers, the latter being preferablyadded during or before the melting step. Further, the step of forming aheated bubble may include a stretching step, which orients the film.

In some embodiments of the disclosed invention, the method may furtherinclude annealing the film. The annealing step may be carried out at atemperature ranging from about 120° F. to about 285° F., preferably fromabout 140° F. to about 250° F. The bubble may be heated to a temperaturegreater than about 100° F.

In another embodiment of the present invention, a method of pretreatinga PLA polymer blend to allow the manufacture of a blown film is providedcomprising (a) providing pellets of a PLA polymer blend, (b) melting thepellets to form a molten mass at a first desired viscosity value orrange of values, and (c) cooling the molten mass to a second desiredviscosity value or range of values in a polymer cooling unit. The seconddesired viscosity value may fall in the range of about two times toabout ten times the first desired viscosity value, and preferably in therange of about four times to about eight times the first desiredviscosity value.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention that will be described below and which will form thesubject matter of the claims appended hereto.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments inaddition to those described and of being practiced and carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein, as well as the abstract, are for thepurpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an assembly for production of PLA blownfilms in accordance with one embodiment of the instant invention.

FIG. 2 is a graph depicting the percent shrinkage of a 200 Gauge film ofthe present invention at a given temperature for ten seconds. Thediamond data points represent shrinkage in the machine direction (MD)and the square data points represent expansion in the transversedirection (TD).

FIG. 3 is a graph depicting the percent shrinkage of a 200 Gauge film ofthe present invention at a given temperature for thirty seconds. Thediamond data points represent shrinkage in the MD and the square datapoints represent expansion in the TD.

FIG. 4 is a graph depicting the percent shrinkage of a 200 Gauge film ofthe present invention at a given temperature for five minutes. Thediamond data points represent shrinkage in the MD and the square datapoints represent expansion in the TD.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the instant invention, plastic films and a methodof their manufacture are described. The polymer films are generatedthat, when heat treated, shrink in the machine direction and expand inthe cross direction. This shrinkage and expansion can occursubstantially simultaneously. The machine direction will be definedherein as the “longitudinal” or “length” direction. The “crossdirection” or “cross web direction” or “transverse direction” will bedefined herein as the direction perpendicular to the machine direction.Embodiments of films described herein exhibit reduced edge-curlingcharacteristics and can be produced in clear, matte, translucent colorsand opaque colors. Films of the present invention may be suitable forback side. and/or front side printing.

Films of the present invention may comprise various polymers and polymergrades known in the art. Preferably, in some embodiments, polymers areselected that, when used alone or in a blend, enable “memory” to bestored from an orienting process described herein. Many of thedeterminants for polymer selection are known to one or ordinary skill inthe art and/or will be apparent from the teachings herein. For example,where high temperature processing is desired, polymers with highersoftening points may be selected, such as, for example, relatively highmolecular weight polymers.

Shrink films of the instant invention comprise PLA, and may optionallyinclude additives known in the art, e.g., antiblock additives, slipadditives, impact modifiers and “viscosity enhancers.” These additivesare generally employed to enhance the processing, performance and lookof the final product as will be discussed below. In each of theaforementioned classes, various grades of the respective polymers areunderstood to be included. Each of these classes of polymers andadditives will now be discussed, in turn, as relevant to the instantinvention.

PLA

Since both lactic acid and lactide can achieve the same repeating unit,the general term polylactic acid as used herein refers to polymershaving the repeating unit of formula I without any limitation as to howthe polymer was made (e.g., from lactides, lactic acid, or oligomers),and without reference to the degree of polymerization.

The polylactide used in the invention may be made from L-, D- orD,L-lactide, or blends thereof, by any polymerization process. A highmolecular weight polymer can be produced by ring-opening polymerizationfrom lactic acid dimer, lactide. Lactic acid is optically active, andthus its dimer appears in four different forms: L,L-lactide;D,D-lactide; L,D-lactide (“mesolactide”); and a racemic mixture of L,L-and D,D-lactides. By polymerizing these dimmers either as pure compoundsor at different blend proportions, polymers are obtained which havedifferent stereochemical structures affecting their resilience andcrystallinity and, consequently, also their mechanical and thermalproperties. The obtained polymers are usually hard and optically clear.

Copolymers or polymer blends may also be used in PLA films of thepresent invention. The weight-average molecular weight (Mw) of polymerssuitable to the invention is approximately 10,000-400,000, preferably40,000-250,000.

Polylactide is in equilibrium with its monomer, lactide. This chemicalproperty can lead to rapid hydrolysis and cause problems of adhesion inthe processing of the polymer. Furthermore, the presence of the monomerlowers thermal stability during melt processing. Therefore, residuallactide is typically and preferably removed from the polymer. Preferablemonomer content is preferably below about 2%, and more preferably belowabout 1%.

Heat, during film processing steps, may also contribute to polymerdegradation. In addition to the removal of lactide monomer, another wayto retard premature hydrolysis of the polymer is to reduce the watercontent of the polymer to below 500 ppm, and more preferably, to below200 ppm. Methods to further reduce and/or maintain low water content aredescribed herein below.

PLA is available from multiple suppliers and the polymers and/or polymerblends of the instant invention are not limited to any one grade orsupplier thereof. However, NatureWorks® polymers, as supplied byCargill, Inc. are preferred in some embodiments of the instant invention(e.g., grades 4060D, 4042D, 4032D). While each of grades 4060D, 4042Dand 4032D has a molecular weight average 200,000 to about 400,000, theyare prepared with differing percentages of D-lactide. Grade 4042D isprepared with about 3 to about 5 weight percent D-lactide. Grade Grade4060D comprises about 11 to about 13 percent D-lactide; grade 4032Dpolymer comprises about 1 to about 2 percent D-lactide. These polymersare supplied with a lactide concentration of lower than about 1 percent,a mesolactide level of about 10 percent to about 20 percent, and amoisture level lower than about 500 ppm.

While the instant invention can be suited with any PLA polymer of anygrade and composition, the concentration of D-lactide, in particular,can affect the physical properties of the resulting PLA polymer. By wayof example, increasing the percent of D-lactide in a polymer or apolymer blend reduces the capacity of the resulting polymer tocrystallize, which, in turn, increases undesirable degradation of thepolymer at higher temperatures. Said another way, lowering the weightpercent of D-lactide in a polymer composition increases the temperatureresistance of the polymer and hence the viscosity of the resulting meltat a given temperature as well.

Polymers and/or polymer blends with higher levels of D-lactide can yieldfilms that begin to shrink at lower temperatures when exposed to heat;these films also tend to exhibit more “gentle” shrink curves, i.e., lessshrinkage per rise in temperature. Conversely, films comprising polymerswith relatively low D-lactide concentration generally require exposureto higher temperatures to shrink. It should also be noted that,typically, PLA polymers with lower concentrations of D-lactide are moreexpensive than otherwise equivalent polymers having greater D-lactide.Therefore, there can be an economic incentive to maximize the use of PLAwith higher levels of D-lactide; however, this incentive should bebalanced with the desired physical properties of the films.

Antiblock Additives

Antiblock (also called “antitack”) additives serve to improve processingand application of polymer films. Specifically, this class of additivesis used to reduce the adhesion between films. Antiblock agents—typicallyfinely divided, solid minerals, but also waxes—act by producing a slightroughening of the surface. Antiblock agents are mainly used in filmextrusion and include natural silica, synthetic silica, talc, calciumcarbonate, and N,N′-Ethylene Bis(Stearamide) (EBS).

Antiblock additives are typically “loaded” with a carrier compound.While it is by no means a requirement, it is preferable that the carrierpolymer be similar to or equivalent to one or all of the polymers in themaster PLA blend. In the instant invention, for example, it is preferredthat the carrier polymer be a PLA polymer. As the “active ingredient” inan antiblock comprises only a small fraction of the final composition,adding a carrier compound provides ease and consistency in measurements.One of ordinary skill in the art would recognize to take theconcentration of filler into account when calculating the finalconcentration of antiblock in the final product. For example, if acomposition comprising 10 percent antiblock consists of 10 percent“active ingredient,” the final concentration of the “active ingredient”is 1.0 percent of the total. The values hereinbelow, including those inTable 1, are provided as a percentage of the “active ingredient” in thefinal formulation.

In the instant invention, the final concentration of antiblock is lessthan about 3 percent by weight, preferably less than about 2 percent byweight, and more preferably less than about 1 percent by weight. In oneembodiment of the present invention, antiblock 2100D from Cargill ispreferred. 2100D comprises 10% talc filled magnesium silicate in grade4032D PLA carrier and has a mean particle size of less than about 1micron.

In selecting an appropriate antiblock agent, the particle size thereofis one factor to consider. Indeed, the particle size of an antiblockagent can directly contribute to the overall smoothness of the resultingfilm. That is, all other factors being equal, a smaller antiblockparticle size will result in a smoother film. Another factor to consideris the concentration of the antiblock. The concentration of antiblock ispreferably minimized particularly, as in some cases, because antiblockadditives can introduce haze to the film. Hence, particularly inapplications where haze is undesirable (e.g., envelope windows, etc.),the concentration of antiblock may be minimized.

Slip Additives

Slip additives are modifiers that act as an internal lubricant to reducethe coefficient of friction (COF) between two overlapping films, forexample, in films rolled after production. Indeed, lower COFs areespecially desirable for film applications. These additives migrate tothe surface of the plastic during and immediately after processing. Thatis, a non-visible coating “blooms” to the surface to provide amicroscopic “layer” of air between two adjacent sheets of film. In thisway, enhanced lubricity and slip characteristics are provided.

Accordingly, slip additives may be considered similar to antiblockadditives in that they both serve to lower the COF between twooverlapping films. Films of the instant invention may comprise one, bothor neither class of additives. Typical slip agents are, for example,oleamide, erucamide, stearamide, behenamide, oleyl palmitamide, stearylerucamide, ethylene bis-oleamide, EBS, including most grades of theirrespective refinement. In some embodiments, EBS is a preferred slipagent, and EBS with 4032D carrier is more preferred. EBS is sold underthe tradenames Advawax, Lubrol EA, and Micotomic 280.

As with antiblock agents, the “active ingredient” of slip additives isgenerally supplied with a carrier. Films of the instant inventioncomprise less than about 1 percent by weight of a slip additive(referring to the “active ingredient” only), and more preferably lessthan about 0.5 percent by weight. It should be noted that excessiveamounts of slip additive may produce films that are excessively smooth,which can compromise the ability of substances (e.g., ink, stickers,etc.) to adhere to the surface. Thus, to enhance, for example, theprinting properties of shrink films of the instant invention, the amountof slip additive may require adjustment accordingly.

Viscosity Enhancers

Although numerous methods are known and available to increase theviscosity of polymers during processing of blown films, the term“viscosity enhancer” is defined herein to encompass any chemical agentthat increases or maintains the viscosity of a polymer at a giventemperature. Viscosity enhancers may be introduced into the polymerblend at any time until the polymer enters the die (discussed below),however, viscosity enhancers are preferably introduced prior toextrusion, and more preferably, during blending of the polymer pellets.

Viscosity enhancers can improve the finished properties of films bypreventing and/or reversing the degradation encountered duringprocessing of polymer films. Some viscosity enhancers are “stabilizers.”That is, they are used in virgin plastic to either (1) protect againstdegradation in processing and/or (2) reverse the degradation caused byrecycling, and return the plastic to nearly its original performanceproperties. Another class of viscosity enhancers, “coupling agents,” forexample, improves the processability of extruded polymer by “coupling”individual polymer strands thereby increasing the melt strength of theplastic.

Viscosity enhancers are generally known and available to one of ordinaryskill in the art and have their broadest application with polyesters,polyamides (nylon) and polycarbonates. It is believed that viscosityenhancers have heretofore never been successfully applied to PLA filmtechnology. Though the chemical identity of viscosity enhancers isgenerally proprietary, the products are available from vendors such asJohnson Polymer LLC (USA) and Clariant International Ltd. (Switzerland).Viscosity enhancers suitable in the instant invention are not limited tothose exemplified and/or those with similar mechanisms. In fact, anychemical agent that increases or maintains the viscosity of a polymer ata given temperature may suffice.

In a preferred embodiment of the invention, shrink films are generatedcomprising a coupling agent for enhancing viscosity. Once such couplingagent, CESA®-extend, is available from Clariant. These viscosityenhancers contain a proprietary copolymer of styrene, methylmethacrylate and glycidyl methacrylate. Without being limited by orbound to theory, CESA®-extend viscosity enhancers are thought to repairthe damage (e.g., polymer breakdown) that heat and moisture can cause toPLA resins by coupling individual PLA polymers. In this way, theviscosity enhancer may “extend” polymer chains in the presence of somedegradation and thereby attenuate overall loss of molecular weight andviscosity of the polymer melt.

Optimum reactivity of CESA®-extend can be achieved with a preferable uselevel of less than about 0.5 percent by weight, and preferably less thanabout 0.25 percent by weight. Again, the “active ingredient” ofviscosity enhancers is generally supplied with a carrier. CESA®-extendviscosity enhancers may undergo a process residence time of about 2 toabout 20 minutes, depending on starting moisture and/or inherentstarting viscosity of the polymer. As well, process residence times arepreferably minimized to attenuate polymer degradation from the heatrequired in the process. A detailed discussion of processing conditionsis provided below. In any event, one of ordinary skill in the art wouldappreciate that it may be necessary to adjust the concentration of anypolymer enhancer based, at least, on some of the factors listed above.

In some embodiments, polymers for films may be selected from one, two oreach of four groups (i.e., PLA, antiblock and slip additives, viscosityenhancers) and combined to create blended polymer films. Table 1 belowprovides non-limiting examples of formulations that may be suitable inthe preparation of films of the present invention. For clarity indescription, a skilled artisan should appreciate from the teachingsherein that the percent of additives calculated is only the “activeingredient.” In other words, while it should be appreciated that theadditives are generally supplied in as a combination of “activeingredient” and carrier, the percent of carrier, if any, has not listed.TABLE 1 Polymer Formulations Composition (percentage by weight) No.4060D 4042D 4032D Antiblock Slip Visc. Enhancer 1 69.58%    0% 30% 0.15%0.18% 0.09% 2 97.50%    0%  0% 1.50% 0.50% 0.50% 3 0% 0% 99.58%   0.15%0.18% 0.09% 4 0% 0% 97.50%   1.50% 0.50% 0.50% 5 0% 98.58%     0% 0.15%0.18% 0.09% 6 0% 98%   0% 1.50% 0.50% 0.50% 7 98.58%    0%  0% 0.15%0.18% 0.09% 8 59.58%    10%  30% 0.15% 0.18% 0.09% 9 49.58%    20%  30%0.15% 0.18% 0.09% 10 39.58%    30%  30% 0.15% 0.18% 0.09% 11 48.61%   30%  20% 1.12% 0.18% 0.09% 12 48%  30%  20% 1.00% 0.50% 0.50% 13 30% 20%  48% 1.00% 0.50% 0.50% 14 30%  30%  39.58%   0.15% 0.18% 0.09% 1530%  0% 68.85%   0.15% 0.50% 0.50% 16 68.85%    0% 30% 0.15% 0.50% 0.50%17 0% 68.85%    30% 0.15% 0.50% 0.50% 18 30%  68.85%     0% 0.15% 0.50%0.50% 19 29.58%    30%  40% 0.15% 0.18% 0.09% 20 70%  0% 29% 0.50% 0.25%0.25% 21 29%  0% 70% 0.50% 0.25% 0.25% 22 99%  0%  0% 0.50% 0.25% 0.25%23 0% 99%   0% 0.50% 0.25% 0.25% 24 0% 0% 99% 0.50% 0.25% 0.25% 25 100% 0%  0%   0%   0%   0% 26 0% 100%   0%   0%   0%   0% 27 0% 0% 100%    0%  0%   0% 28 0% 0%  0%  100%   0%   0% 29 99.63%    0%  0% 0.10% 0.18%0.09% 30 0% 99.63%     0% 0.10% 0.18% 0.09% 31 0% 0% 99.63%   0.10%0.18% 0.09% 32 99.68%    0%  0% 0.05% 0.18% 0.09% 33 0% 99.68%     0%0.05% 0.18% 0.09% 34 0% 0% 99.68%   0.05% 0.18% 0.09% 35 30%  30% 39.68%   0.05% 0.18% 0.09% 36 39.68%    30%  30% 0.05% 0.18% 0.09% 37 0%39.68%    30% 0.05% 0.18% 0.09%

The blends in Table 1 may be chosen or prepared to create the “feel” orflexibility of the film to match an end-use application. Where all otherprocessing parameters are equal, the ratio of PLA polymers may beadjusted in accordance with the teachings of the instant invention toprovide shrink films with desirable physical properties. For example,reducing the concentration of D-lactide (i.e., increasing weight precent4032D) yields polymers with a higher softening point and films thatrequire greater temperature to shrink, which moves the shrink curve tothe right.

In a preferred embodiment, films are produced with a blend of about 98weight percent PLA polymer and less than about 2 percent additives byweight and in which the PLA polymer component is about 68 percent 4060DPLA and 30 percent 4032D by weight. Shrink films comprising formulationno. 1 in Table 1 is more preferred.

Impact Modifiers

While practice of the present invention does not require the use ofimpact modifies (a.k.a. “plasticizers”), their use may be beneficial inprocessing. For example, plasticizers can reduce brittleness. Manyplasticizers are known in the art and the present invention is notlimited in their use herein. Non-limiting examples include: 1924X, 1901Xand 1657G (Kraton® Polymers); Biomax® 4024 and Biomax® D 4026 (DuPont®);Nodax™ (Procter & Gamble); P209, P240 and P226 (BioMer, Canada); andBionelle® 1000 (Showa HighPolymer Co., Ltd.). Typically, the “activeingredient” of an impact modifier is an acrylate polymer, co-polymer orderivatives thereof, (e.g., ethylene acrylate and/or other synthetic or“natural” polymers, commonly referred to as “elastomers.” Non-limitingexamples of elastomers include butadiene and isoprene polymers, bothbranched or linear. As with the other additives noted above,plasticizers are generally supplied with a carrier. For example, theplasticizer butadiene may be supplied in polystyrene carrier, and insome instances, the carrier can independently provide plasticizingfunction.

The net amount of plasticizer (i.e., free of carrier) is preferably lessthan about by weight 5%, more preferably less than about 4%, and mostpreferably less than about 3% of the total polymer blend. In someembodiments the amount of plasticizer can range from about 1.5% to about3.5% by weight. Table 2 below provides non-limiting examples offormulations that may be suitable in the preparation of inventive filmscomprising plasticizer. TABLE 2 Polymer Formulations ComprisingPlasticizer Composition (percentage by weight) No. 4060D 4042D 4032DPlasticizer Antiblock Slip Visc. Enhancer 1 69.58% 0% 28.25%   1.75%  0.15% 0.18% 0.09% 2 69.58% 0% 27.5%   2.5%   0.15% 0.18% 0.09% 3 69.58%0% 27% 3% 0.15% 0.18% 0.09% 4 69.58% 0% 26.5%   3.5%   0.15% 0.18% 0.09%5 67.58% 0% 30% 2% 0.15% 0.18% 0.09% 6 66.58% 0% 30% 3% 0.15% 0.18%0.09% 7 59.58% 8% 30% 2% 0.15% 0.18% 0.09% 8 59.58% 7% 30% 3% 0.15%0.18% 0.09% 9 68.85% 0% 27.5%   2.5%   0.15% 0.50% 0.50% 10 68.85% 0%26.5%   3.5%   0.15% 0.50% 0.50% 11   29% 0% 68% 2% 0.50% 0.25% 0.25% 12  29% 0% 67% 3% 0.50% 0.25% 0.25%Processing

After the polymer composition of the film is selected, the polymer isthen processed to generate a film with desirable shrink properties.Generally, the polymers are procured in pellets or grains. In caseswhere multiple polymers are to be included, the polymers pellets arefirst dry blended. That is, the pellets are mixed together. In apreferred embodiment of the invention, the pellets are then processedinto film by blown film technology. PLA blown films and methods ofmanufacturing same are described in U.S. Provisional Patent ApplicationNos. 60/605,151 and 60/609,827 filed Aug. 30, 2004 and Sep. 15, 2004,respectively, the disclosures of which are incorporated herein in theirentirety by reference.

Blown film processing can be characterized in five steps: extrusion,temperature conditioning, orienting, collapsing. A preliminary step ofdrying the polymer pellets is preferable, but not required. As well, aterminal step of annealing may be preferable, but not required accordingto the instant teachings. An assembly for each processing step isgenerally depicted in FIG. 1 and will now be described in detail.

Drying

PLA polymers are generally supplied in sealed bags from the manufacturerand in relatively dry condition. Typically, the moisture content ofthese as-supplied PLA polymers is less than about 500 ppm and preferablyless than about 200 ppm. Where the moisture level is deemed desirable,no further drying may be necessary or required. However, PLA readilyabsorbs moisture from the atmosphere and therefore, the blended polymerpellets are optionally and preferably first dried by heating in a dryerto remove surface moisture. Without being bound by or limited to theory,it is believed that the removal of moisture content may help control therelative viscosity loss due to hydrolysis. As mentioned above, highertemperatures and the presence of even a small amount of moisture canhydrolyze PLA in the ensuing melt phase.

PLA is generally produced by a reversible condensation reaction, whichproduces water; when undried PLA is heated, hydrolysis can occur and keymechanical properties of the PLA may be compromised. For example, theviscosity of the polymer, when melted, is inversely proportional to thepercentage of free monomer therein. Therefore, in an attempt to minimizebatch-to-batch variation in viscosity, preferably, significant moistureis removed from the polymer pellets. In some embodiments, a moisturecontent of less than about 200 ppm is preferable, and less than about 50ppm, more preferable (measured by the Karl Fisher method).

A dehumidifying hopper with hot air at a relatively low dew point may beused; however, a variety of air dryers are known in the art and many ofthem may be suitable for drying. The present invention need not belimited to air dryers only, but may include other types of dryers,including baking ovens. A dehumidifying hopper may be desirable in someembodiments in that dehumidified air passes through a bed of PLA toextract moisture from the resin. A desiccant material, such as silica,absorbs moisture from the circulating air. Dual desiccant bed systemsare common, so that one bed is on-stream while a stand-by bed is beingregenerated. Either a time-cycle or a predetermined decrease in air dewpoint is used to shift airflow from one bed to the other. Suchmethodology is thought to be effective in removing some moisture thatmay reside below the surface of the polymer pellets in addition to thesurface moisture.

Preferable dryers of the instant invention for drying PLA may have oneor more of the following characteristics:

-   1. Desiccant beds capable of achieving a dew point of about −40° C.    in the supply air-   2. A means, e.g., an after-cooling unit, to eliminate or reduce the    likelihood of a temperature spike in the supply air-   3. Superior temperature control in the PLA drying range

The temperature and duration of drying may be dependent on the totalamount and condition of the polymer(s) (i.e., the amount of startingsurface moisture), and may need to be adjusted on a batch-by-batchbasis. Preferably, the polymers experience little to no melting in thisstep. By way of example, typical drying conditions require thattemperatures range from about 110° F. to about 230° F., and preferablyfrom about 130° F. to about 190° F. for variable periods of time. By wayof specific example, the residence time for drying polymer with air (dewpoint, −40° F.) at a flow rate of greater than about 0.5 ft3/min.requires about 4 hours at about 110° F. and about 2 hours at about 190°F. Higher drying temperatures may lead to softening and blocking ofpolymer, while lower drying temperatures can result in extended dryingtimes and/or incomplete drying.

Dew point is an absolute measure of air moisture and is independent ofair temperature. Dew point may be used to control dryer performance.Airflow is another component to drying, as it heats the resin andabsorbs its moisture. Sufficient airflow can maintain the resin at theproper temperature for its entire residence time. In embodiments wereadditional colorants, additives, or otherwise ingredients are used, itmay be preferable to minimize moisture-related degradation by furtherdrying same.

Extrusion

Extrusion is whereby the pellets are melted into a low viscosity moltenmass, thus combining the heretofore individual polymer beads or grainsinto one molten mass. The viscosity of the melt will depend on thetemperature. Temperatures can range from about the temperature at whichthe polymers will remain melted to about the temperature wheredegradation of the polymers begins to occur. By way of example,extrusion melt temperatures may be maintained between about 325° F. toabout 485° F. for certain PLA polymer blends, but may ultimately dependon the different polymers that have been blended and their respectivemelting points. In some embodiments, about 400° F., is preferred.

By way of example, the viscosity of PLA at about 480° F. and an apparentsheer rate of about 5.5 seconds⁻¹ in a capillary rheometer may rangefrom about 1,000 poise (P, dyne/cm²) to about 8,000 P, preferably about3,000 P to about 6,000 P, and more preferably, about 4,500 P. At a shearrate of about 55 seconds⁻¹, the same polymer at about 480° F. may havean apparent viscosity that ranges from about 1,000 P to about 5,000 P,preferably about 2,000 P to about 4,000 P, and more preferably, about3,000 P.

Temperature Conditioning

Temperature conditioning is done to increase the viscosity of the moltenpolymers, which makes the melt manageable for further processing.Indeed, it should be noted that one significant aspect of the instantinvention provides a method of changing the viscosity of the polymermelt upon extrusion and before orientation (i.e., before the polymerexits the die). While the cooling step may be accomplished by a varietyof methods known in the art, the use of a polymer cooler is one means tothis end. The viscosity of the polymer melt may also be adjusted, aloneor in combination for example, by air cooling the die inner mandrelthrough which the polymer film is blown, the use of viscosity enhancersnoted above, and combinations of these techniques.

The use of a polymer cooler at this step in processing may enable moreprecise temperature control than by air cooling the die alone. It isbelieved that temperature control over the orienting process whichfollows the cooling step allows for shrink memory to be stored in thefilm.

A variety of coolers are known in the art and may be used by one ofordinary skill in the art based on the teaching provided herein.However, a Koch Engineering SMR polymer cooling unit, available fromSulzer Chemtech, USA of Tulsa, Okla. adapted for PLA use may bepreferred in some applications. By “adapted,” it is meant that a polymercooler may have to be adjusted for a cooling capacity lower than thatfor polystyrene, for example. In other terms, the pressure in theprimary loop for polystyrene cooling is generally about 1000 psi toabout 7,000 psi and, in some instances, about 5,000 psi; by contrast,the pressure in the same loop adjusted for PLA use may range from about300 psi to about 4,000 psi.

The polymer cooler operating temperature range is preferably betweenabout 280° F. to about 450° F. Higher temperatures may be used, but suchhigher temperatures may also contribute to degradation of the polymer.The temperature and duration of cooling can again depend on both theamount of polymer being cooled and the film properties that may bedesired. In one example, the viscosity of PLA at 375° F. and an apparentsheer rate of about 5.5 seconds⁻¹ in a capillary remoter, may range fromabout 15,000 P to about 17,000 P, preferably about 15,500 P to about16,500 P, and more preferably, about 16,000 P. At a shear rate of about55 seconds⁻¹, the same polymer at 375° F. may have an apparent viscositythat ranges from about 14,000 P to about 16,000 P, preferably about16,500 P to about 15,500 P, and more preferably, about 15,000 P. It willbe apparent from the data presented herein that the polymer cooling stepcan increase the viscosity from about 2 to about 10 times that of thepolymer coming out of the extruder. In other embodiments, the viscositymay be increased about 5 to about 9 times.

The polymers demonstrate a substantial increase in viscosity uponcooling in the polymer cooler, which cooling procedure, in part, isthought to allow for subsequent blowing of the film. It is also apparentthat the viscosity of the PLA polymers exhibits a consistent shearviscosity of a relatively large range of shear rates at any giventemperature.

Orienting

The next step in preparation of films of the present invention isorienting, also known as stretching. This step imparts the shrink“memory” into the film where it is “stored” by the polymer blend.Orienting can be accomplished by many methods and associated equipmentknown to one of ordinary skill in the art, including, for example,machine/cross direction orientation and blown film orientation. Allmethods are preferably designed to first control the temperature of thepolymer, followed by a controlled stretching operation. Without beinglimited to or bound by theory, it is believed that the orienting processconveys strength and flexibility to the film product. Furthermore,though orientation bubbles may be pulled both up or down (i.e.,vertically) or horizontally through a die, it may be preferable to pullsaid bubble upward to facilitate control and maintenance of the polymertemperature during orientation.

In a preferred embodiment of the present invention, the polymer melt isalready pre-cooled, preferably in a polymer cooler, and then submittedto a blown film orientation process. The process of the presentinvention has at least one significant advantage in that a verycontrolled temperature—from the post extrusion temperatureconditioning—can be achieved prior to the formation of a bubble. A blownfilm extrusion process extrudes molten plastic polymer through a die ofcircular cross-section and uses an air jet to inflate a bubblecomprising same.

In the preferred embodiment then, by virtue of pre-cooling the meltedpolymer, only a final fine tuning of orienting temperature is performed,where desired, during the orientation process. In other words, thegreater share of temperature conditioning takes place prior to orientingand not during orienting. Where a fine tuning of temperature is desired,it can be relatively easily accomplished by a temperature controlled airring, which blows chilled air at the base of the bubble.

Die parameters may range from 1:0.75 BUR (Blown Up Ratio) to about 1:2.0BUR, and preferably, about 1:1.4 BUR in the cross web direction. In thelength direction, die parameters may range from about 1:1 draw downratio to about 1:300 draw down ratio, and preferably, about 1:190 drawdown ratio. Orienting temperatures of the present invention range fromabout 100° F. to about 180° F., and more preferably, about 140° F.

Collapsing

Once the extrudate has been inflated into a circular bubble, it then is“collapsed” into a double thickness film. The collapsing process isperformed by use of an “A-frame,” also known as a collapsing frame. Thisframe uses rollers, panels, and/or flat sticks to flatten the bubbleinto a sheet of double-thickness film (FIG. 1). The sheets areultimately cut and wound onto two finished rolls, or coils, of PLA film.The sheets of film can also be cut to desired length.

In accordance with another teaching of the present invention, it hasalso been learned that control of the film temperature while in bubbleform may prevent the formation of undesirable wrinkles and/or filmlayers that stick together upon passage through the collapsing niprollers. By control, it is meant that the temperature of the polymerbubble is preferably maintained at a temperature greater than about 100°F., more preferably the temperature is maintained at a range from about100° F. to about 200° F., and even more preferably, from about 120° F.to about 140° F.

The temperature of the polymer bubble may be regulated by a variety ofmethods. In one embodiment, it has been found that it may be beneficialto include a heated oven (FIG. 1) enclosure constructed around thecollapsing frame—and generally extending around a portion of thebubble—to control the temperature at which the bubble is collapsed. Theoven enclosure may optionally extend to and be sealed at or near the topof the bubble to better maintain insulation and temperature control. The“oven” may generally comprise any device that prevents the polymerbubble from cooling below a predetermined temperature, and may includeboth heated panels and/or insulation alone. In a preferred embodiment,the oven comprises a heat source preferably located at or near the topof the collapsing frame. The heat generated therefrom is then maintainedand circulated within the oven by virtue of insulation encompassing thebubble.

In yet another embodiment, the temperature of the polymer bubble can beregulated by means of internal circulation of warm (e.g., about 150° F.)air through said bubble. Of course, it should be understood that aheated oven may be used in conjunction with any means for internalcirculation of warm air.

Annealing

Annealing, also called crystallization or relaxation, is typically thefinal step in the preparation of films of the instant invention.According to the teachings herein, an annealing step is optional. Whendesired, annealing is generally accomplished post-orienting, andperformed at temperatures between about 120° F. to about 285° F. in someembodiments. This process is accomplished by rotating heated cylindersthat contact the film just prior to the winding process where thefinished roll of plastic film is generated.

Film properties using the aforementioned protocol can be manipulated asdesired with nominal trial and error by one of ordinary skill in theart. Such variations are expected and are incorporated into the scope ofthe invention. Films of the instant invention can generally havecharacteristics that fall into the following ranges:

Film Shrinkage at about 95° C. for about 10 seconds:

-   -   Longitudinal direction: about 10% to about 90%, with an average        of about 50%.    -   Cross direction: about 0% to about −30% (growth), with an        average of about −5%.

In one embodiment of the instant invention, percent shrinkage as afunction of temperature was studied with, for example, 200 gauge (0.002in.) film subjected to 10 second, 30 second, and 5 minutes exposure toheat (see FIGS. 2, 3 and 4, respectively). The films tested herein wereprepared from a blend of about 68.53% grade 4060D PLA, about 30% grade4032D PLA, about 1.2% grade 2100D antiblock of which 10% is “activeingredient,” about 0.18% slip EBS and about 0.09% polymer resin chainextender CESA®-extend. (The term “about” has been used herein andthroughout this specification to account for the customary variations inmeasurements common and expected by one of ordinary skill practicingthis art.) Using the preferred process described above, the polymerblend, without additional drying, was extruded at about 390° F., andcooled in a polymer cooler to below about 375° F. The cooled polymer wassubsequently blown film oriented between about 120° F. and 140° F. andfinally annealed at about 195° F. Die ratios were 1:1.4 in the cross webdirection and 1:190 in the machine direction.

The respective films were placed in a hot water bath at the temperatureshown for the indicated times, and the shrinkage in the machinedirection (MD) and the transverse direction (TD) as a percentage of theoriginal dimensions was plotted. While the shrink films studied in thisexample were subjected to heated water, exposure to heat in any form(e.g., heated air from a hot air dryer) may induce shrinkage of theinventive films. As is evident from the results, the films describedexhibit positive shrinkage in the MD and negative shrinkage (i.e.,growth or expansion) in the TD at all the temperatures tested.

This growth in the cross direction can prevent the label from curlingback on the edges during the shrinking process. Accordingly, in oneembodiment, films of the present invention may have application inroll-to-roll (i.e., wrap around) labeling of various shaped cylindersand cones such as, for example, batteries, cans, bottles, disposablelighters, pens, floral wraps and other decorative items. However, thescope of applications should not be limited to the aforementionedconsumer products or uses.

Films of the present invention have innumerable other applications. Forexample, these films can also be used in printed/unprinted applicationfor holding together twin packs, attachments, neck bands, and perforatedneck bands for decoration or tamper evident use, to name a few options.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention, which fallwithin the spirit and scope of the invention. Further, since numerousmodifications and variations will readily occur to those skilled in theart, it is not desired to limit the invention to the exact constructionand operation illustrated and described, and accordingly, all suitablemodifications and equivalents may be resorted to, fall within the scopeof the invention.

1. A shrink film comprising a polylactic acid polymer blend which, uponexposure to heat, exhibits shrinkage in the length direction and growthin the cross direction.
 2. The shrink film of claim 1 in which the filmexhibits shrinkage in the length direction in the range of about 10% toabout 90% and exhibits a shrinkage in the cross direction in the rangeof about 0% to about −30% (a negative value indicates growth) whensubjected to 95° C. heat for about 10 seconds.
 3. The shrink film ofclaim 2 in which the film exhibits shrinkage in the length direction ofabout 50% and exhibits a shrinkage in the cross direction of about −10%(a negative value indicates growth) when subjected to 95° C. heat forabout 10 seconds.
 4. The shrink film of claim 2 in which the filmexhibits shrinkage in the length direction of about 30% and exhibits ashrinkage in the cross direction of about −5% (a negative valueindicates growth) when subjected to heat 95° C. heat for about 10seconds.
 5. The shrink film of claim 2 in which the film exhibitsshrinkage in the length direction of up to about 50% or greater andexhibits a shrinkage in the cross direction of up to about −10% (anegative value indicates growth) when subjected to heat over atemperature range of about 60° C. to about 95° C. for about 10 seconds.6. The shrink film of claim 1 further comprising one or more additives.7. The shrink film of claim 6 in which the one or more additives is aantiblock additive, a slip additive, a viscosity enhancer, an impactmodifier or a combination thereof.
 8. The shrink film of claim 7 inwhich the antiblock additive is natural silica, synthetic silica, talc,talc filled magnesium, calcium carbonate, and N,N′-EthyleneBis(Stearamide) (EBS).
 9. The shrink film of claim 8 in which theantiblock additive is talc filled magnesium.
 10. The shrink film ofclaim 6 in which the slip additive is oleamide, erucamide, stearamide,behenamide, oleyl palmitamide, stearyl erucamide, ethylene bis-oleamide,EBS, or a combination thereof.
 11. The shrink film of claim 10 in whichthe slip additive is EBS.
 12. The shrink film of claim 7 in which theviscosity enhancer is a stabilizer or a coupling agent.
 13. The shrinkfilm of claim 12 in which the coupling agent is CESA®-extend.
 14. Theshrink film of claim 1 in which the PLA polymer blend includes two ormore grades of PLA polymer.
 15. The shrink film of claim 6 in which thePLA polymer grades are about 1 to about 2 percent by weight D-lactide,about 3 to about 5 percent by weight D-lactide and about 11 to about 13percent by weight D-lactide.
 16. The shrink film of claim 6 comprisingabout 50 weight percent to about 90 weight percent of PLA polymer havingabout 11 to about 13 percent by weight D-lactide; and about 10 weightpercent to about 50 weight percent of a PLA polymer having about 1 toabout 2 percent by weight D-lactide.
 17. The shrink film of claim 7comprising about 60 weight percent to about 80 weight percent of PLApolymer having about 11 to about 13 percent by weight D-lactide; andabout 20 weight percent to about 40 weight percent of a PLA polymerhaving about 1 to about 2 percent by weight D-lactide.
 18. The shrinkfilm of claim 8 comprising about 65 weight percent to about 75 weightpercent of PLA polymer having about 11 to about 13 percent by weightD-lactide; and about 25 weight percent to about 35 weight percent of aPLA polymer having about 1 to about 2 percent by weight D-lactide. 19.The shrink film of claim 14 further comprising an antiblock additive, aslip additive, a viscosity enhancer or a combination thereof.
 20. Theshrink film of claim 19 comprising about 50 weight percent to about 90weight percent of PLA polymer having about 11 to about 13 percent byweight D-lactide; about 10 weight percent to about 50 weight percent ofa PLA polymer having about 1 to about 2 percent by weight D-lactide;less than about 3 percent by weight of an antiblock additive; less thanabout 1 percent of a slip additive; and less than about 0.5 percent of aviscosity enhancer.
 21. The shrink film of claim 20 comprising about 50weight percent to about 90 weight percent of PLA polymer having about 11to about 13 percent by weight D-lactide; about 10 weight percent toabout 50 weight percent of a PLA polymer having about 1 to about 2percent by weight D-lactide; less than about 2 percent by weight of anantiblock additive; less than about 0.5 percent of a slip additive; andless than about 0.25 percent of a viscosity enhancer.
 22. The shrinkfilm of claim 21 comprising about 50 weight percent to about 90 weightpercent of PLA polymer having about 11 to about 13 percent by weightD-lactide; about 10 weight percent to about 50 weight percent of a PLApolymer having about 1 to about 2 percent by weight D-lactide; less thanabout 1 percent by weight of an antiblock additive; less than about 0.25percent of a slip additive; and less than about 0.1 percent of aviscosity enhancer.
 23. A packaged good comprising a consumable item anda heat treated PLA shrink film wrapped around at least a portion of theconsumable item, which heat treated film is obtained by exposing to heata PLA shrink film comprised of a PLA polymer blend which exhibitsshrinkage in the length direction and growth in the cross direction uponexposure to heat.
 24. A method of making a PLA shrink film, whichexhibits shrinkage in the length direction and growth in the crossdirection, comprising (a) providing dry pellets of PLA, (b) melting thepellets to form a molten mass at a first desired viscosity value orrange of values, (c) increasing the viscosity of the molten mass to asecond desired viscosity value or range of values, (d) forming a heatedbubble from the resulting molten mass, and (e) collapsing the bubble toform a film.
 25. A method of pretreating a PLA polymer blend to allowthe manufacture of a shrink film, which exhibits shrinkage in the lengthdirection and growth in the cross direction, comprising (a) providingpellets of a PLA polymer blend, (b) providing one or more viscosityenhancers, (b) melting the pellets and one or more viscosity enhancersto form a molten mass at a first desired viscosity value or range ofvalues, and (c) cooling the molten mass to a second desired viscosityvalue or range of values.